$-\dfrac{3}{5} + \dfrac{1}{4} = {?}$
Answer: $ = - {\dfrac{3 \times 4}{5 \times 4}} + {\dfrac{1 \times 5}{4 \times 5}} $ $ = - {\dfrac{12}{20}} + {\dfrac{5}{20}} $ $ = - \dfrac{{12} + {5}}{20} $ $ = -\dfrac{7}{20}$